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In the late 23rd Century, with the Federation refitting their Consitution-class starships and the Klingons introducing the K'T'inga-class Battlecruisers, the Romulans had to remain competitive and refitted the D7s they purchased from the Klingons during their brief alliance in the mid 23rd Century. They removed the Shuttlebay and the Impulse Engine housing above the Engineering Deck and moved it below, between the warp nacelles for "better efficiency".

Note: This does appear to be simlar to the DC Star Trek comic "Time Crime", where there was a D7 with a similar structure. But it was not inspired by it in any shape or form.

The idea came about as a possible way the Romulans evolved their ships from the Warbirds we saw in Enterprise and in TOS, where they based ther 24th Century ships off the Klingon D7 and "improved it", to where it eventually evolves into the D'Deridex and the Valdore (Mogai-class).

They essentially moved the Impulse Drive from the D7 and moved it between the Warp Nacelles in their belief of warfare efficency. This allowed the main engineering to be further armored to protect the Cloaking Device and Warp Core (this was before they mastered singularities). So while the Klingon D7's evolved into the K'T'inga as an offensive warship, the Romulan D7's would evolve into more of a defensive / hit and run ship that's typical of Romulan Warbirds.